The present invention generally relates to vehicle brake systems and more particularly relates to a brake force booster for automotive vehicles.
A brake force booster of this type is generally disclosed in EP 655 039. The valve member of this brake force booster defines in the control housing a pneumatic chamber which is adapted to be acted upon by the pneumatic pressure that prevails in the working chamber. For this purpose, a pneumatic channel is provided between the pneumatic chamber and the working chamber in the area of a sealing surface of the valve member.
The prior art brake force booster has principally a satisfactory operational performance because the pneumatic chamber permits achieving a reduction of the difference between the reaction force of the brake force booster and the resetting force acting on the valve piston so that an increase of the resetting force is possible with the reaction force remaining constant, or a reduction of the reaction force is possible with the resetting force remaining constant. This improves the hysteresis of the brake force booster of the present invention.
However, the principally positive operational performance of the brake force booster does not cover all operating conditions. For example, with the brake force booster having reached its maximum attainable boosting effect, the valve member tends to remain in this maximum boosting effect condition because the resulting force which is applied to the valve member due to the pneumatic chamber, assists in the separation of the vacuum channel so that the valve member continues to be urged in the direction of the sealing seat. Consequently, the valve member is not pressure-balanced to full extent in all operating conditions.
Therefore, an object of the present invention is to provide a brake force booster with an ideally pressure-balanced valve member which takes into account the cost reductions demands in the automotive supplier industry, hence, in particular permitting ease of manufacture and assembly of the individual parts, and effectively utilizing the limited mounting space.
This object is achieved by the present invention wherein the valve member includes at least two sealing lips of different diameters which define the pneumatic chamber in the control housing, and the effective pneumatic surface of the chamber which is influenced by the difference in diameters is generally in conformity with the pneumatic surface defined between the two sealing seats.
A complete pressure balance of the control valve is achieved with these features in a simple way of manufacture, without requiring complicated constructions with difficult assembly operations.
It is favorable that the sealing lips largely align with the sealing seats in an axial direction. This effects a space-saving construction in a radial direction and a small deviation of the air flows during ventilation or evacuation, which permits a good and quick response behavior of the brake force booster.
According to the preferred embodiment, a guide element with two concentric guiding sockets is associated with the valve member with the sealing lips. Each of the sealing lips acts resiliently on a contact surface of a socket. The advantage achieved by this measure is that the valve member is guided exclusively on a component in an axial direction, other than in the state of the art where it is guided on a valve housing and a guide element, which necessitates a very precise fabrication with close tolerances, due to the addition of tolerance fields.